1. Field of the Invention
The present invention relates to a fan drive device for drive control of a fan drive motor and, more particularly, to the fan drive device having a detection circuit for detecting rotation of a fan drive motor caused by external air flow such as, for example, wind causing the fan drive motor in an outdoor unit of an air conditioner to turn.
2. Description of Related Art
Systems for detecting the wind-driven rotation of a fan drive motor in the outdoor unit of an air conditioning system are known in literature. A typical circuit diagram of a conventional fan drive device for controlling fan drive motor drive in such an outdoor unit is shown in FIG. 9. As shown in FIG. 9, this fan drive device 100 comprises a drive circuit 102 for driving a fan drive motor 110 such as found in the outdoor unit of an air conditioning system in one direction; a control circuit 103 for controlling the drive circuit 102; and a voltage monitoring circuit 104 for monitoring the output voltage of the dc power circuit 111 serving as a power source for the fan drive motor 110. The drive circuit 102 has six n-channel MOS transistors, referred to below as simply MOS transistors. The control circuit 103 controls the rotational speed of the fan drive motor 110 by PWM control of the MOS transistors. The drive circuit 102 and control circuit 103 together form a so-called inverter.
When the fan drive motor 110 is not driven by the fan drive device 100, the fan drive motor 110 can be driven by the wind in the direction opposite to the direction of normal rotation of the fan drive motor, that is, in reverse rotation. As the wind becomes stronger, the reverse rotation speed of the motor rises. If the wind is sufficiently strong, it is even possible without driving the fan drive motor 110 to achieve the air flow required for the heat exchange operation of the outdoor unit of the air conditioning system. It is also possible, however, to damage the fan drive device 100 and fan drive motor 110 when the electric power is supplied to the fan drive motor 110 to drive the latter in the direction of normal forward rotation while the fan drive motor 110 is being reverse-driven by the wind. This potential damage may be typically prevented by controlling the fan drive device 100 to not drive the fan drive motor 110 when the fan drive motor 110 is turning in the reverse direction at a speed exceeding a predetermined threshold.
A prior art method of detecting the direction of fan drive motor 110 rotation is described below.
When the fan drive motor 110 is driven by an external force, induction voltage is produced in the fan drive motor 110. This induction voltage is then converted to dc by a diode in the drive circuit 102. The voltage monitoring circuit 104 detects and monitors this converted dc voltage. If the voltage detected by the voltage monitoring circuit 104 exceeds a threshold value when the fan drive motor 110 is to be started, the control circuit 103 controls the drive circuit 102 so that the fan drive motor 110 is not started.
The fan drive motor induction voltage detected by the voltage monitoring circuit 104 in this method is, however, affected by the magnetization strength of the rotor magnet in the fan drive motor 110, as well as variations in such components as the frequency dividing resistors used for frequency dividing the induction voltage in the voltage monitoring circuit 104. This creates a problem with the induction voltage detection precision of the voltage monitoring circuit 104, and the precision of fan drive motor speed detection.
The dc power circuit 111 supplies current to the compressor (not shown) as well as the fan drive motor 110, and therefore uses high capacity electrolytic smoothing capacitors 115. As a result, when the fan drive motor 110 is reverse-driven by the wind and produces an induction voltage, a relatively long period of time is required for the dc voltage detected by the voltage monitoring circuit 104 to stabilize because more time is required to charge these high capacity electrolytic smoothing capacitors 115.
The reliability and efficiency of the voltage monitoring circuit 104 is also not optimal because high voltage is normally applied to the voltage monitoring circuit 104 and the internal frequency dividing circuit formed by the frequency dividing resistors. This is a particular problem when the voltage monitoring circuit 104 detects fan drive motor speed using the above-described method if the fan drive motor 110 is PWM driven with a high voltage.